Coated ferrite RF filters

ABSTRACT

In a low pass RF filter, a coating of barium titanate is applied to a ferrite substrate. In one embodiment, the RF filter is an extruded tube of ferrite coated with barium titanate. The tube is used as an RF filter for a connector pin. In another embodiment, a thin strip of ferrite is coated with barium titanate. This forms a filter strip for use on circuit boards or for use as a high capacity lossy power bus.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my prior copendingapplication Ser. No. 883,501, filed Dec. 9, 1969 now abandoned, and towhich priority is asserted as to subject matter common therewith.

BACKGROUND OF THE INVENTION

This invention relates to low pass RF filters and more particularly to alayer of dielectric material deposited on a ferrite substrate to form afilter.

Low pass RF filters are used extensively in electrical circuits tosuppress stray radio frequency noise. Lumped impedance filters performwell at the lower frequencies but resonances limit their utility as thefrequency is increased. Also, these type filters are large in sizecompared to the circuits with which they are used. To overcome this, RFfilters of the type disclosed in U.S. Pat. No. 3,275,953 -- Coda et al.were developed and used as feed through filters or on connector pins.These filters are small and have good insertion loss characteristics athigh frequencies. However, there are several problems associated withfilters of the type shown in the Coda et al. patent. First, they includean inner sleeve of ferrite coated with a metal layer and an outermetallized ceramic sleeve, usually barium titanate. Therefore, theyrequire several fabrication steps. Also, the capacity is limited by thethickness to which the outer sleeve can be made, usually 8 to 10 milsminimum.

Finally, even though resonances are minimized at high frequencies, thefilter, because of the type of construction, is still .[.lumpted.]..Iadd.lumped .Iaddend.at the lower frequencies of interest, 1-50megahertz. Accordingly, resonances can result at these frequencies. Itis desirable then that filters of this type have a completelydistributed impedance, that they be easier to fabricate and that theynot be limited in capacity by the titanate sleeve thickness.

SUMMARY OF THE INVENTION

This invention concerns an RF filter in which a thin coating ofdielectric material is laid down on a ferrite substrate. In one specificembodiment a layer approximately 2 mils thick of barium titanate is laiddown on the ferrite substrate to produce an electrical filter. Thefilter produced in this manner has low cost because fewer fabricationsteps are involved. Also, the electrical properties are better thanprior art filters. The impedance is completely distributed and thefilter has a high capacity.

In one form of the invention, the filter is an extruded tube of ferriteupon which a layer of barium titanate has been deposited. These filtersare used for connector pins.

In another form of the invention the filter is a thin strip of ferriteupon which a barium titanate layer has been deposited. These are used asfilter strips, or filtered buses, for circuit boards.

The foregoing and other objects, features and advantages of theinvention will be better understood from the following more detaileddescription, the drawings, and the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a prior art type of filter for a connector pin;

FIG. 2 depicts a filter for a connector pin constructed in accordancewith the present invention;

FIG. 3a shows the insertion loss versus frequency for the prior artfilter and for the filter of this invention;

FIG. 3b shows the attenuation versus frequency for the prior art filterand for the filter of this invention;

FIG. 4 shows the equivalent circuit of a prior art type filter;

FIG. 5 shows the filter of the present invention in place on a connectorpin;

FIG. 6 shows the invention embodied in a filter strip for a circuitboard; and

FIGS. 7-9 show modifications of the filter strip.

DESCRIPTION OF A PARTICULAR EMBODIMENT

Referring to FIG. 1, most prior art connector pin filters areconstructed of two concentric sleeves. The inner sleeve includes anextruded ferrite tube 1 with metal plating 2. The outer sleeve includesbarium titanate 3 with the metal plate 4. The two sleeves are joinedtogether by conductive epoxy or by soldering. It will be appreciatedthat the fabrication process includes extruding two sleeves, two stepsof plating, one for each sleeve, and the step of joining the two sleevestogether.

Contrast this with a filter constructed in accordance with the presentinvention as depicted in FIG. 2. The extruded ferrite tube 5 is coatedwith the barium titanate layer 6. Barium titanate may be laid down onferrite with several known techniques. Electrophoretic deposition is aparticularly good technique for coating barium titanate on a ferritetube. The electrophoretic deposition described in Senderoff et al. U.S.Pat. No. 2,843,541 may be used to lay down the barium titanate layer.After the barium titanate has been deposited on the ferrite, the deviceis metal plated, the metal plating being indicated at 7. Gaps 8 and 9are left in the metal plating to isolate the ground and center pinelectrodes.

Note that in FIG. 1 the same provision must be made for gaps 10 and 11in the metal plating. Additionally, a gap 12 must be provided on theinside of sleeve 3. When the inner sleeve 1 is electroplated, a gap 12amust be provided so that the ferrite is not shielded out of the circuit.It can be seen that the fabrication of prior art devices includeadditional difficult fabrication steps not required in constructing thefilter of this invention.

The filter of this invention also has improved electricalcharacteristics over prior art filters. This can best be shown by anexample. Two filters were constructed, one in accordance with the priorart and one in accordance with this invention. The filters were 0.1inches in diameter by 0.465 inches long. The capacity of the prior artfilter was 6,000 μμF. The capacity of the filter of this invention was5,000 μμF. The insertion loss versus frequency of the filters asmeasured in a 50 ohm system is shown in FIG. 3a. This response for bothfilters is good. Note, however, that the attenuation of the two filters,shown in FIG. 36, is quite different and that the prior art filteractually shows an undesirable gain between 5 and 10 megahertz. This gainis the result of the shunt capacity resonating with the filter seriesinductance. These circuit elements are shown in the equivalent circuitof FIG. 4. Because the impedance of the circuit in which these filterdevices are used is not always known or easily established, the priorart filter in actual use may show less loss or even a gain from thatdetermined by measurement made in a circuit with predetermined sourceand load impedance, such as mil standard 220. The filter of thisinvention, because of its distributed construction and inherent low Q,does not show gain in the attenuation curve of FIG. 3b, regardless ofthe circuit impedances.

As previously pointed out, another advantage of this invention is thatit is possible to get an extremely thin film of barium titanate, about 2to 4 mils being common. This thin film gives a much higher capacity perunit length of filter and for a given dielectric constant there is moreattenuation per unit length than in a conventional filter.

FIG. 5 shows one of the filters constructed in accordance with theinvention in place on a connector pin. The filter 13 is positioned overthe connector pin 14. A ground plane 15 is snapped onto the filter toprovide the ground connection. For a connector pin filter having alength of 1 centimeter, the noise attenuation is approximately 60 db at100 megahertz. That is, the noise power is reduced by a factor of 10⁶.

FIG. 6 shows an embodiment of the invention in which a coated strip offerrite is used as a filter strip for a circuit board. The strip offerrite 16 has a layer of barium titanate 17 deposited thereon. Thestrip has a metal plating 18 on one side and a metal plating 19 on theother side. The metal plating 19 is soldered to the ground plane 20 ofthe circuit board. The circuit components 21 and 22 are connected to theRF filter strip by the leads 23 and 24 respectively. These leads aresoldered to the metal plating 18.

In this form, the invention provides good filtering for componentsconnected to the metal plating 18 which may be a DC bus. Particularly indigital circuits, when one of the circuits such as 21 or 22 istriggered, high frequencies are normally imposed on the DC bus. Thishigh frequency noise may interfere with the other circuits on the board.However, the use of the lossy bus according to the present inventionwill isolate the components. Also, it will prevent noise from othersources from entering the power bus and possibly causing a malfunction.

In one actual application of the invention a 1/2 inch wide bus bar wasconstructed of the form shown in FIG. 6. The attenuation was 90 db percentimeter at 100 megahertz. Stated another way, the power of the noisewas attenuated by a factor of 10⁹.

Many modifications of the invention will be apparent. While bariumtitanate has been described as a particularly good dielectric material,other dielectric materials with lower dielectric constants may bedeposited on the ferrite as a means of controlling the cut off frequencyof the filter. For example, a filter with a 2 mil epoxy coating resultedin a filter with a cut off frequency (frequency at which insertion lossis 3db) of 50 megahertz, whereas the equivalent filter with bariumtitanate had a cut off at 2 megahertz.

Various modifications of the filter strip may be made. For example, inFIG. 7 the ferrite 25 has coatings of barium titanate 26 and 27 on bothsides. Conductive metal platings 28 and 29 are applied over the bariumtitanate. Such a filter strip has a higher breakdown voltage. However,it would also have lower attenuation for a given thickness of bariumtitanate.

In FIG. 8 the layers 30, 31, and 32 are conductive metal coatings.Layers 33 and 34 are ferrite and layers 35 and 36 are barium titanate.The metal 31 may be the conductor and the metal coatings 30 and 32 theground. This embodiment has a higher capacity and loss per unit length.

In FIG. 9 the filter strip includes a single ground conductor 36, aferrite 37 and a barium titanate layer 38. Laid down on this are aplurality of metal conduction strips 39-42. This embodiment can be usedwhere multiple circuits are required.

While the invention is particularly suitable for use with a ferritesubstrate as previously described, the substrate may, in accordance witha further aspect of the invention, be constructed of other materials.One practical alternative in the use of a doped semiconducting ceramicmaterial for the substrate. It is well known that the normally highresistivity of barium titanates can be greatly reduced by theintroduction of proper additives. The resulting semiconductive bariumtitanates, produced by known methods of treatment referred to in U.S.Pat. No. 3,268,783 to Osamu Saburi, are termed "controlled valencysemiconductive barium titanates." As is pointed out in the Saburipatent, semiconductive ceramic material can be produced via valencycontrol and can be carried out upon members of the family of materialsgenerically designated by E²⁺ M⁴⁺ O₃ ²⁻, wherein E is an alkaline earthelement material selected from the group consisting of barium,magnesium, calcium, strontium, lead and mixtures thereof, M is a metalchosen from the group consisting of titanium, tin, and zirconium, and Ois of course oxygen. Barium titanate is one member of the aforesaidfamily of materials. As further pointed out in the Saburi patent, theadditives used for valence control may comprise a material A selectedfrom the group consisting of yttrium, actinium, thorium, antimony,bismuth, the members of the rare earth elements, and mixtures thereof,or a material B taken from the group consisting of vanadium, niobium,tantalum, selenium, tellurium, tungsten, and mixtures thereof. The totalamount of additive should be between 0.01 atomic percent to 0.50 atomicpercent of the host material, the alkaline earth material E being thehost with additive A, and the metal M being the host in the case ofadditive B. In FIGS. 1 and 2 of the Saburi patent, the semiconductiveplate 3 is an illustration of a semiconductive barium titanate substratein a capacitor device.

In accordance with this further aspect of the invention, the substratemay consist of a semiconducting ceramic, such as the aforementionedsemiconductive barium titanate, which is then coated with a suitabledielectric material to produce a filter, the coating being deposited inthe same manner as in the case of the ferrite substrate above. Forexample, a semiconducting barium titanate sleeve coated with a lowconductivity titanate forms a large lossy capacitor. Such a device doesnot have the loss characteristics associated with the magnetic ferriteand it is not as effective as the ferrite device at high frequenciesHowever, for some applications the filter constructed with asemiconducting ceramic substrate is quite satisfactory and can beinexpensively manufactured.

What is claimed is:
 1. A .[.unitary.]. .Iadd.composite ceramic.Iaddend.low pass filter element for mounting on a conductor of a lowfrequency transmission line to attenuate high frequencies thereoncomprising,a conductive tubular member for receiving a conductortherein, a semiconductive substrate in the form of a sleeve secured tothe outer surface of the tubular member in intimate engagementtherewith, a .Iadd.ceramic dielectric .Iaddend.layer .[.of dielectricmaterial.]. covering the outer surface of the sleeve in direct intimatecontact therewith, and an outer conductive layer disposed about andsecured to the dielectric .[.material.]. .Iadd.layer.Iaddend.substantially the length thereof for connecting the unitaryfilter element to ground.
 2. The filter recited in claim 1 wherein said.[.substrate.]. .Iadd.semiconductive substrate .Iaddend.is a.[.semi-conducting.]. ceramic.
 3. The filter recited in claim 2 whereinthe .[.semiconducting ceramic.]. .Iadd.semiconductive substrate.Iaddend.is doped barium titanate.
 4. The filter recited in claim 1wherein said dielectric is undoped barium titanate.
 5. The filterrecited in claim 1 wherein the layer of dielectric material is coatedthereon.
 6. The filter recited in claim 1 wherein the tubular member isa metallic plating over the inner surface of the sleeve.
 7. The filterrecited in claim 5 wherein the outer conductive layer is metallicplating over said layer of dielectric material.
 8. A .[.unitary.]..Iadd.composite ceramic .Iaddend.low pass filter strip for mounting on acircuit board provided with a ground plane conductor comprising,asubstrate in the form of a flat strip of semiconductive material, aconductive metal plating on one surface of the substrate, a coating of.Iadd.ceramic .Iaddend.dielectric material on the opposite surface ofthe substrate, and a conductive metal plating on the outer surface ofthe dielectric coating, one of said platings being connected to oneterminal of a low frequency source and load, and the other plating beingin conductive engagement with the ground plane conductor.
 9. The filterof claim 8 in which the substrate is a semiconductive ceramic.
 10. Thefilter of claim 8 in which the substrate is doped barium titanate. 11.The filter of claim 8 in which the dielectric is undoped bariumtitanate.
 12. A .[.unitary.]. .Iadd.composite ceramic .Iaddend.low passfilter element for mounting on a conductor of a low frequencytransmission line to attenuate high frequencies thereon comprising,aconductive tubular member for receiving a conductor therein, a substrateof ferrite in the form of a sleeve secured to the outer surface of thetubular member, in intimate engagement therewith, a layer of.Iadd.ceramic .Iaddend.dielectric material covering the outer surface ofthe sleeve in direct intimate contact therewith, and an outer conductivelayer disposed about and secured to the dielectric materialsubstantially the length thereof for connecting the unitary filterelement to the ground.
 13. The filter recited in claim 12 wherein saiddielectric is undoped barium titanate. .Iadd.
 14. The filter of claim 8in which the substrate is a ferrite..Iaddend.